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What is a Solder wire

Solder is basically a metallic compound that has a low melting point. The composition of solder varies depending on the type, but usually it is a mix of tin and lead. For soldering in electronics, solder is mostly available in wire form. For some very fine soldering jobs including soldering of miniature surface-mount components it is recommended to use a solder paste. Solder wire is silvery colored wire that is usually supplied on reels. It is a very flexible and very soft and is easily cut by wire cutters. Solder wire that comes in smaller diameters can even be cracked by hand.

Standard lead-based solder wire

Most common tin-lead proportions in solder wire are 60Sn/40Pb (for 60% tin, 40% lead by weight) and 63Sn/37Pb (for 63% tin, 37% lead by weight). I have used both 60/40 and 60/37 for years with no issue. More lead in solder alloy means a lower melting point of solder because lead has a lower melting point than tin. Melting point of solder should be low enough to be melted with a soldering iron. Melting point of solder is usually around 200°C - to be more accurate 63Sn/37Pb  solder has a melting point of 183°C, slightly lower than the more common 60Sn/40Pb solder which has has a melting point of around 190°C.  

Lead-free solder

The European Union passed law in 2003 stating that no equipment sold in Europe should, after July 2006, have more than 0.1% lead in any homogenous component (like a solder joint). This law is known as the WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restriction of Hazardous Substances). So far, there are no US laws (outside California) mandating the removal of lead, however most manufacturers are switching to lead-free solder for competitive reasons. Therefore, solder containing lead is slowly being phased out under these European Union directives and being replaced with lead-free solders. The main reason why European Union banned use of Standard lead-based solder is a concern that the lead from those electronics will leach into ground water supplies from landfills.
Lead is not present in solder fumes to any considerable degree (these fumes actually comes from flux and are still bad for your health). Lead does not get absorbed through the skin. Main risk of soldering with lead comes from ingesting lead by eating or smoking without first washing hands
Lead does not get absorbed through the skin, and is actually not present in solder fumes to any appreciable degree (fumes are still bad for you, see fumes section below). The greatest risk of hand soldering with lead comes from ingesting lead by eating or smoking without first washing
Lead-free solder has a higher melting point than lead-based solder – melting point is at around 230°C. Lead-free solder uses more aggressive fluxes. That means that the iron bits need a different coating to withstand the flux. In addition, lead-free solder is about 20%-50% more expensive than standard lead-based solder.

Diameter of solder wire

Most of solder wires used for soldering of electronic components and printed circuit boards come in diameters 0.020”- 0.050”. Solder wire 0.020” is very thin solder wire and is recommended for hand-soldering of small components or fine surface-mount parts. Solder wire 0.050” is a thick solder wire and is recommended for soldering of big joints, like heavy gauge wire or leads. Mid-sized solder wire 0.025” – 0.031” is recommended for most soldering work in electronics.